Integrated circuit design typically begins with the knowledge of a design goal or specification. Such specifications describe the required performance of the circuit(s) to be designed. Integrated circuit designers typically first create a computer model of a circuit design that will meet the performance specification. That circuit design model is based upon the electrical circuit elements that are available in a given semiconductor manufacturing process. For example, electrical circuit elements may include diodes, transistors, capacitors, resistors, and so on. Typical modern integrated circuits include both p-channel field effect transistors and n-channel field effect transistors. Integrated circuits that include both p-channel field effect transistors and n-channel field effect transistors are referred to as CMOS integrated circuits.
It is common in semiconductor process technology nodes from about 45 nm and beyond, to find that a substantial amount of static power in CMOS integrated circuits is attributable to off-state leakage current in the transistors. The power consumption that results from this unwanted leakage current is undesirable, and consequently designers seek to reduce the power consumption due to leakage current.
One approach to reducing leakage current is to re-design the circuitry of an integrated circuit. Regardless of whether such a re-design can reduce leakage current without reducing performance, a high cost will be incurred in re-tooling the design. As is known in the field of integrated circuit design, the cost of preparing masks and fabricating integrated circuits with those mask sets is very high. In view of the high cost of re-designing an integrated circuit, designers desire to re-use their circuit designs (both transistor network topologies and physical layout). Circuit design re-use may reduce non-recurring engineering costs and may also reduce the time required to implement an integrated circuit design in fully fabricated integrated circuits. Re-using an integrated circuit design generally includes re-using the physical layout for that design. Re-using a physical layout generally means fabricating an integrated circuit in the same process for which it was originally designed, or fabricating it in an alternative process with different electrical characteristics but substantially the same feature sizes.
Unfortunately, the performance of many integrated circuit designs is very sensitive to complex combinations and distributions of variations in the electrical characteristics of transistors resulting from uncontrolled variations in the semiconductor manufacturing process. In some instances these performance sensitivities can result in a circuit that does not meet its specified performance goals. Failure to meet performance goals may result in the binning of parts into lower performance categories, and may even lead to total failure. Such yield losses are of great concern to designers.
It is noted that the cross-sectional representations of various semiconductor structures shown in the figures are not necessarily drawn to scale, but rather, as is the practice in this field, drawn to promote a clear understanding of the structures, process steps, and operations which they are illustrating.